The present invention relates to a telltale device for perishable goods and more particularly to a device for the visual indication that a pre-selected temperature has been reached or exceeded for a predetermined time.
Many materials, such as for example, film, pharmaceutical and biological products and the like, must be stored under prescribed conditions of temperature in order to retain the freshness or effectiveness of the product. If the storage temperature or critical temperature of the product is exceeded for a definite period of time, the material will deteriorate or otherwise be adversely affected. In addition, many products, even if stored at or below their critical temperature, will deteriorate or lose freshness and should not be used after a prescribed period of time. Often, however, the appearance of the product is such that the consumer or user cannot determine whether the product has been properly stored or is still in a useful condition at the time of purchase or selection for use.
Various temperature indicators or telltales have been developed to indicate the temperature history of a product. Typically, these devices include a temperature sensitive material which flows at or above a pre-selected temperature for contact with a suitable indicator material to visually indicate that the carrier has been in a fused state. In some cases, mechanical means, such as a ball or the like, is supported in a temperature sensitive material and upon fusion the mechanical means changes position to provide a positive indication that a temperature has been exceeded.
These devices, however, can only indicate a change of state due to temperature. However, temperature alone is not ordinarily determinative of a product's condition and normally, it is the total time above a critical temperature which determines the condition of a product. Thus, for example, photographic film will deteriorate over a period of time even if properly stored and it is substantially impossible to determine the freshness of the film without actually using it, often with disastrous results.
Some prior art telltale devices have been designed to relate time and temperature. One such device is disclosed in U.S. Pat. 2,782,749 and is based on the use of temperature sensitive materials having non-specific melting points, such as fats and waxes, which gradually become less viscous as temperature increases. These temperature sensitive materials are formed as a layer on a porous paper barrier and at or above a predetermined temperature, the fat or wax begins to diffuse through the paper barrier and eventually reaches a point of visibility on the opposite side of the barrier. The rate of diffusion of the material through the barrier is directly related to the temperature. Devices of this type, due to the nature of fats and waxes, have a relatively narrow temperature range at which they can be used and it is difficult to control the diffusion rate through the paper barrier. In addition, the paper barrier must be relatively thick, on the order of 0.75 inches, so that the device is relatively bulky, thus giving rise to handling and storage problems when attached to packages or containers.
The present invention overcomes the deficiencies of the prior art and provides an improved telltale for product packages and the like for indicating the period of time at which the package and its contents have been at or have exceeded a predetermined temperature. The device of the present invention is adaptable for use through a wide range of temperatures and time periods.